Calcium carbonate (CaCO3) is a calcium salt of carbonic acid, which is widely used in many industries today. It is mostly known as a calcium supplement, taken to increase daily calcium intake. Calcium carbonate has six known polymorphs, three of which are anhydrous crystalline, namely, calcite, aragonite and vaterite; two are crystalline hydrates, namely, monohydrocalcite and ikaite; and one is hydrated amorphous, namely amorphous calcium carbonate (ACC). ACC is a transient polymorph that precipitates out of a supersaturated solution following Ostwald's step rule. If not stabilized by any means, ACC will rapidly and completely crystallize into one of the five more stable polymorphs within seconds. The amorphous polymorph is characterized by distinctive 40-120 nm spherules, having no major XRD peaks but a broad low intensity peak between 20-30 2θ, and having a broad low intensity peak around 1082 cm−1 in Raman spectroscopy, in contrast to the 1-10 μm crystals typical of the other polymorphs, also having distinct major XRD peaks and significantly distinguishable Raman peaks.
Synthetic ACC is known for over 100 years, and today there are many methods for synthesizing ACC using various molecules for stabilizing the transient unstable amorphous phase. The three widely used methods all use supersaturated solution of calcium ions from either a soluble source such as calcium chloride or from dissolving a calcium insoluble salt such as calcium hydroxide using a hydrogen binding molecule, such as sucrose. This supersaturated solution of calcium ions is then reacted with a source of carbonate from either carbon dioxide gas, an alkaline metal salt of carbonate, such as sodium carbonate, from an organic salt of carbonate, from ammonium carbonate, or from the hydrolysis of dialkyl carbonate, such as dimethyl carbonate with hydroxide ions (see, for example, U.S. Pat. No. 4,237,147).
Since ACC is unstable in aqueous solution for more than two minutes, commercial production is impractical. Large scale production that includes hundreds or even thousands of liters being mixed and separated using liquid-solid phase separation techniques, such as filtration or centrifugation, in less than two minutes, is not applicable today. If the stability time in solution can be prolonged to several hours, therefore allowing for standard liquid-solid phase separation techniques, such as filtration or centrifugation to be used, commercial production can then become practical.
With the exception of Hyun et al. [Materials Chemistry and Physics, 93 (2005) 376-382], that described a method to stabilize ACC in ethanolic medium for more than 24 hours, none of the above previous reports mention the period of time in which the ACC remains stable in solution. However, Hyun et al. can only produce stable ACC in the presence of toxic ammonia, which, as described by Hyun, is crucial to the stability. Also, the calcium carbonate concentrations used in the publication are relatively low, making them impractical for industrial use.
When attempting to reproduce other published procedures, the applicants of the present invention produced ACC that is only stable in solution for several minutes and crystallizes thereafter. In some cases, even though ACC was produced, it was impossible to isolate it from the solution. For instance, producing ACC using the procedure described in U.S. Pat. No. 4,237,147 at Example 2 yielded only a slurry that was impossible to filter and from which ACC could not be isolated. Also, should a powder be obtained from this slurry using spray drying, as suggested in this patent, it will only contain ˜2/15 of ACC, with the remaining 13/15 parts being sucrose.
In general, any attempts to duplicate the procedures described in U.S. Pat. No. 4,237,147 using calcium chloride, or some other soluble calcium salt did not yield ACC or any form of precipitated calcium carbonate.
It is well known that ACC will crystallize in the presence of water, however, to the applicant's best knowledge, there are no previous publications describing the production of ACC which remains stable in aqueous solution or suspension for extensive periods of time using only up to 10% by weight of stabilizers. Also, the carbonation step in all these methods is the last step of the synthesis, always followed by the liquid solid separation step.
There is an unmet need in the art for novel methods for producing ACC with increased stability, either as a suspension in aqueous phase, or as a dry powder, which can be adapted to production of ACC on commercial production scale.